Tunable composite longitudinal vibration mechanical filter manufacturing method

ABSTRACT

A composite longitudinal vibration mechanical filter for delivering out a supplied high-frequency signal in a predetermined frequency range includes a plurality of vibratable bodies including input and output longitudinal vibratable tuning bars with piezoelectric members superposed thereon, coupling elements by which the longitudinally vibratable tuning bars are coupled to each other, supporting elements projecting respectively from the input and output longitudinally vibratable tuning bars, and a holder to which the supporting elements are attached. At least one resonant frequency adjusting finger is disposed on at least one of the vibratable bodies. Grooves are defined in the longitudinally vibratable tuning bars at the same time that they are fabricated. The grooves extend in the direction in which the longitudinally vibratable tuning bars are longitudinally vibratable, and are shorter than the length of the longitudinally vibratable tuning bars. Through holes may instead be defined in the longitudinally vibratable tuning bars, the through holes having an opening size smaller than the wavelength of the longitudinal vibration of the longitudinally vibratable tuning bars. Piezoelectric members are fixedly superposed on the input and output longitudinally vibratable tuning bars in sandwiching relation thereto.

This application is a continuation of application Ser. No. 07/971,481,filed Nov. 4, 1992 (abandoned), which is a division of application Ser.No. 07/483,454, filed Feb. 21, 1990 (now U.S. Pat. No. 5,187,458).

BACKGROUND OF THE INVENTION

The present invention relates to a composite longitudinal vibrationmechanical filter which comprises longitudinally vibratable bodies(hereinafter also referred to as "longitudinally vibratable tuningbars"), piezoelectric elements, coupling elements, and supportingelements, and which is capable of appropriately reducing frequencyfluctuations due to different lengths of the longitudinally vibratabletuning bars, undesired spurious responses, and passband deteriorations,or of adjusting the resonant frequency, when desired frequencycharacteristics are created by the transmission of compositelongitudinal vibration, and a method of manufacturing such a compositelongitudinal vibration mechanical filter.

Recently, mechanical filters having characteristics which are of anintermediate level as compared with those of LC filters and quartzfilters are widely used in communication devices. Such mechanicalfilters have a good Q factor, a good selectivity, and a good temperaturecharacteristic, and can be reduced in size.

One conventional composite longitudinal vibration mechanical filter isshown in FIG. 1 of the accompanying drawings. The mechanical filter hasan input longitudinally vibratable tuning bar 2 and an outputlongitudinally vibratable tuning bar 4 which are disposed in the sameplane and are made of a metal material. Identity elastic couplingelements 6, 8 are joined to the input and output longitudinallyvibratable tuning bars 2, 4, and supporting elements 10, 12 projectoutwardly from the centers of the tuning bars 2, 4. The tuning bars 2,the coupling elements 6, 8, and the supporting elements 10, 12 arefabricated by precision pressing and joined together by laser welding orthe like. A pair of input piezoelectric ceramic members 14a, 14b issuperposed on and fixed to the input longitudinally vibratable tuningbar 2 by soldering or the like, and similarly a pair of outputpiezoelectric ceramic members 16a, 16b is superposed on and fixed to theoutput longitudinally vibratable tuning bar 4 by soldering or the like.The supporting members 10, 12 have outer ends secured to upper centralsurfaces of upstanding members 24a, 24b, respectively, of a holder 24 bylaser welding or the like.

A feed line 18 and a grounding line 18e, between which an input signalis supplied, are connected to the input piezoelectric ceramic members14a, 14b and the upstanding member 24a, respectively. Likewise, anoutlet line 20 and a grounding line 20e, from which an output signal isled out, are connected to the output piezoelectric ceramic members 16a,16b and the upstanding member 24b, respectively.

The input and output longitudinally vibratable tuning bars 2, 4, whichare coupled to each other by the coupling elements 6, 8, are held inspace so that they can be longitudinally vibrated unobstructedly. Thecomposite longitudinal vibration mechanical filter is housed in a casing(not shown), which is mounted in an intermediate frequency amplifier ina communication device or the like.

The composite longitudinal vibration mechanical filter shown in FIG. 1operates as follows: A high-frequency signal S₁ produced by a signalsource Osc is supplied to a resistor R and then fed to feed line 18 andthe grounding line 18e, and applied to electrodes (not shown) attachedto the input piezoelectric ceramic members 14a, 14b. The appliedhigh-frequency signal S₁ generates an electric field having the samefrequency as that of the signal S₁, between the electrodes and the inputlongitudinally vibratable tuning bar 2 which is electrically grounded.In response to the electric field thus generated, the inputpiezoelectric ceramic members 14a, 14b are mechanically deformed in thedirections indicated by the arrows Vm, Vn in FIG. 1, and the inputlongitudinally vibratable tuning bar 2 resonates to produce alongitudinal wave having a frequency F₁ and a half wavelength which isequal to the length L₁ of the input longitudinally vibratable tuning bar2. If the longitudinal wave propagates along the input longitudinallyvibratable tuning bar 2 at an average speed V, then the frequency F.sub.1 is given by the following equation:

    F.sub.1 =V/(2L.sub.1)                                      (1)

The longitudinal vibration of the input longitudinally vibratable tuningbar 2 is mechanically coupled and propagated to the outputlongitudinally vibratable tuning bar 4 to the coupling elements 6, 8,causing the output longitudinally vibratable tuning bar 4 to resonate orvibrate longitudinally at a frequency F₂ and with a half wavelengthequal to the length L₂ of the tuning bar 4. If the longitudinal wave ispropagated to the output longitudinally vibratable tuning bar 4 at anaverage speed V, then the frequency F₂ is given by the followingequation:

    F.sub.2 =V/(2L.sub.2)                                      (2)

The longitudinal vibration of the output longitudinally vibratabletuning bar 4 produces a voltage between the output piezoelectric ceramicmembers 16a, 16b. The produced voltage is then led out between theoutlet line 20 and the grounding line 20e as a high-frequency signal S2having a sharp frequency characteristic curve.

In the process of manufacturing the composite longitudinal vibrationmechanical filter shown in FIG. 1, much importance is attached to theaccuracy of a central frequency and the bandpass characteristics of theproduced mechanical filter, and it is desired that the resonantfrequencies F₁, F₂ of the input and output longitudinally vibratabletuning bars 2, 4 have the same central frequency. However, since theinput and output longitudinally vibratable tuning bars 2, 4 aremass-produced in large quantities by etching or precision pressing, itis difficult to give the individual components a sufficient level ofdimensional accuracy. As a result, the mass-produced mechanical filtershave different central frequencies and relatively poor bandpasscharacteristics.

The feed line 18 and the outlet line 20 are spaced from each other toreduce the inductive coupling therebetween due to a stray capacitance,i.e., to increase the isolation therebetween. However, since anundesired vibratory wave which is produced by the input longitudinallyvibratable tuning bar 2 is transmitted to the output longitudinallyvibratable tuning bar 4 via the coupling elements 6, 8 and thesupporting elements 10, 12, unwanted spurious responses are createdoutside of the passband of the mechanical filter.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a compositelongitudinal vibration mechanical filter which has improved frequencycharacteristics, can well be mass-produced, and has desired passband andspurious response characteristics, and a method of manufacturing such acomposite longitudinal vibration mechanical filter.

Another object of the present invention is to provide a compositelongitudinal vibration mechanical filter which has grooves defined invibratable bodies by etching or the like at the same time that thevibratable bodies are fabricated, for thereby allowing the mass-producedvibratable bodies to vibrate at a highly accurate central frequency, andwhich effectively prevents frequency fluctuations from occurring forthereby enhancing and uniformizing passband characteristics, so that themechanical filter can well be mass-produced, and a method ofmanufacturing such a composite longitudinal vibration mechanical filter.

Still another object of the present invention is to provide a compositelongitudinal vibration mechanical filter which has through holes orblind holes defined in vibratable bodies by etching or the like at thesame time that the vibratable bodies are fabricated, for therebyallowing the mass-produced vibratable bodies to vibrate at a highlyaccurate central frequency, and which effectively prevents frequencyfluctuations from occurring for thereby enhancing and uniformizingpassband characteristics, so that the mechanical filter can well bemass-produced, and a method of manufacturing such a compositelongitudinal vibration mechanical filter.

Yet another object of the present invention is to provide a compositelongitudinal vibration mechanical filter which has a highly accuratecentral frequency and has enhanced and uniformized characteristicsinside and outside of the passband thereof, so that the mechanicalfilter can well be mass-produced, and a method of manufacturing such acomposite longitudinal vibration mechanical filter.

Yet still another object of the present invention is to provide acomposite longitudinal vibration mechanical filter which can suppress anunwanted vibratory wave that would otherwise be transmitted from aninput longitudinally vibratable tuning bar via supporting elements andholding elements to an output longitudinally vibratable tuning bar, forthereby effectively reducing an undesired spurious response outside ofthe passband thereof, so that the passband characteristics are improved,and a method of manufacturing such a composite longitudinal vibrationmechanical filter.

A further object of the present invention is to provide a compositelongitudinal vibration mechanical filter which has a highly accuratecentral frequency and which can easily be adjusted after itscharacteristics have been measured, so that the passband characteristicswill be improved, and a method of manufacturing such a compositelongitudinal vibration mechanical filter.

A still further object of the present invention is to provide acomposite longitudinal vibration mechanical filter which has a highlyaccurate central frequency and also has grooves defined in and shorterthan vibratable bodies in order to improve passband characteristicsthereof, and a method of manufacturing such a composite longitudinalvibration mechanical filter.

A yet further object of the present invention is to provide a compositelongitudinal vibration mechanical filter which has a highly accuratecentral frequency and also has through and/or blind holes defined invibratable bodies in order to improve passband characteristics thereof,and a method of manufacturing such a composite longitudinal vibrationmechanical filter.

A yet still further object of the present invention is to provide acomposite longitudinal vibration mechanical filter which has a highlyaccurate central frequency and also has improved characteristics insideand outside of the passband thereof, and a method of manufacturing sucha composite longitudinal vibration mechanical filter.

It is also an object of the present invention to provide a method ofmanufacturing a composite longitudinal vibration mechanical filterincluding a plurality of vibratable bodies including input and outputvibratable bodies with piezoelectric members superposed thereon,coupling elements by which the vibratable bodies are coupled to eachother, supporting elements projecting respectively from the input andoutput vibratable bodies, and a holder to which the supporting elementsare attached, the method comprising the steps of defining grooves in atleast one of the vibratable bodies including the input and outputvibratable bodies at the same time the vibratable bodies are fabricated,the grooves extending in the direction in which the vibratable bodiesare longitudinally vibratable and being shorter than the length of thevibratable bodies, and superposing piezoelectric members fixedly on theinput and output vibratable bodies in sandwiching relation thereto.

Another object of the present invention is to provide a method ofmanufacturing a composite longitudinal vibration mechanical filterincluding a plurality of vibratable bodies including input and outputvibratable bodies with piezoelectric members superposed thereon,coupling elements by which the vibratable bodies are coupled to eachother, supporting elements projecting respectively from the input andoutput vibratable bodies, and a holder to which the supporting elementsare attached, the method comprising the steps of defining through and/orblind holes in at least one of the vibratable bodies at the same timethe vibratable bodies are fabricated, the holes having an opening sizesmaller than the wavelength of the longitudinal vibration of thevibratable bodies, and superposing piezoelectric members fixedly on theinput and output vibratable bodies in sandwiching relation thereto.

Still another object of the present invention is to provide a method ofmanufacturing a composite longitudinal vibration mechanical filter fordelivering out a supplied high-frequency signal in a predeterminedfrequency range, the composite longitudinal vibration mechanical filterincluding a plurality of vibratable bodies including input and outputvibratable bodies with piezoelectric members superposed thereon,coupling elements by which the vibratable bodies are coupled to eachother, supporting elements projecting respectively from the input andoutput vibratable bodies, and a holder to which the supporting elementsare attached, the method comprising the step of fabricating thevibratable bodies which are longitudinally vibratable in a range closeto a passband of the mechanical filter, and the coupling elements whichare disposed between ends of the vibratable bodies and coupled theretothrough flexural vibration, as a unitary structure from a single flatsheet according to a photolithographic process.

Yet another object of the present invention is to provide a compositelongitudinal vibration mechanical filter for delivering out a suppliedhigh-frequency signal in a predetermined frequency range, comprising aplurality of vibratable bodies including input and output vibratablebodies with piezoelectric members superposed thereon, coupling elementsby which the vibratable bodies are coupled to each other, supportingelements projecting respectively from the input and output vibratablebodies, a holder to which ends of the supporting elements are attached,vibration absorbing body holders disposed between opposite ends of thesupporting elements, and vibration absorbing bodies fixedly mounted onthe vibration absorbing body holders.

Yet still another object of the present invention is to provide acomposite longitudinal vibration mechanical filter for delivering out asupplied high-frequency signal in a predetermined frequency range,comprising a plurality of vibratable bodies including input and outputvibratable bodies with piezoelectric members superposed thereon,coupling elements by which the vibratable bodies are coupled to eachother, supporting elements projecting respectively from the input andoutput vibratable bodies, a holder to which ends of the supportingelements are attached, and resonant frequency adjusting fingers disposedon at least one of the vibratable bodies.

A further object of the present invention is to provide a compositelongitudinal vibration mechanical filter for delivering out a suppliedhigh-frequency signal in a predetermined frequency range, comprising aplurality of vibratable bodies including input and output vibratablebodies with piezoelectric members superposed thereon, coupling elementsby which the vibratable bodies are coupled to each other, supportingelements projecting respectively from the input and output vibratablebodies, a holder to which ends of the supporting elements are attached,and at least one of the vibratable bodies having a through groove, ablind groove, a straight groove, a curved groove, or a groove in theform of a combination of these grooves, defined therein and extending ina direction in which the vibratable body is longitudinally vibratable,the groove being shorter than the length of the vibratable body.

A still further object of the present invention is to provide acomposite longitudinal vibration mechanical filter for delivering out asupplied high-frequency signal in a predetermined frequency range,comprising, a plurality of vibratable bodies including input and outputvibratable bodies with piezoelectric members superposed thereon,coupling elements by which the vibratable bodies are coupled to eachother, supporting elements projecting respectively from the input andoutput vibratable bodies, a holder to which ends of the supportingelements are attached, and at least one of the vibratable bodies havingthrough holes and/or blind holes defined therein and having openingsizes smaller than the wavelength of longitudinal vibration of thevibratable body.

A yet still further object of the present invention is to provide acomposite longitudinal vibration mechanical filter for delivering out asupplied high-frequency signal in a predetermined frequency range,comprising a plurality of vibratable bodies including input and outputvibratable bodies for receiving and delivering a high-frequency signal,the vibratable bodies being longitudinally vibratable in a range closeto a passband of the mechanical filter, piezoelectric members superposedon the input and output vibratable bodies, respectively, and havingrespective electrodes connected to conductors, and a plurality ofcoupling elements disposed between ends of the vibratable bodies andcoupled thereto through flexural vibration.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which preferredembodiments of the present invention are shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a conventional composite longitudinalvibration mechanical filter;

FIGS. 2(a), 2(b), 2(c) and 2(d) are perspective views showing thesequence of a method of manufacturing a composite longitudinal vibrationmechanical filter according to an embodiment of the present invention;

FIGS. 3 and 4 are perspective views of composite longitudinal vibrationmechanical filters which are manufactured by the method shown in FIGS.2(a), 2(b), 2(c) and 2(d);

FIGS. 5(a), 5(b), 5(c) and 5(d) are perspective views showing thesequence of a method of manufacturing a composite longitudinal vibrationmechanical filter according to another embodiment of the presentinvention;

FIG. 6(a) is a perspective view of a composite longitudinal vibrationmechanical filter which is manufactured by the method shown in FIGS.5(a), 5(b), 5(c) and 5(d);

FIG. 6(b) is an enlarged fragmentary perspective view of a portion ofthe composite longitudinal vibration mechanical filter shown in FIG.6(a);

FIG. 7 is a perspective view of another composite longitudinal vibrationmechanical filter which is manufactured by the method shown in FIGS.5(a), 5(b), 5(c) and 5(d);

FIGS. 8(a), 8(b), 8(c) and 8(d) are perspective views showing thesequence of a method of manufacturing a composite longitudinal vibrationmechanical filter according to still another embodiment of the presentinvention;

FIG. 9 is a perspective views of a composite longitudinal vibrationmechanical filter which is manufactured by the method shown in FIGS.8(a), 8(b), 8(c) and 8(d);

FIG. 10 is a perspective view of another composite longitudinalvibration mechanical filter which is manufactured by the method shown inFIGS. 8(a), 8(b), 8(c) and 8(d);

FIG. 11 is a perspective view of a composite longitudinal vibrationmechanical filter according to another embodiment of the presentinvention;

FIG. 12 is a perspective view of a composite longitudinal vibrationmechanical filter according to a modification of the mechanical filtershown in FIG. 11;

FIG. 13 is a diagram showing the passband characteristics of themechanical filter shown in FIG. 11;

FIG. 14 is a perspective view of a composite longitudinal vibrationmechanical filter according to a further embodiment of the presentinvention;

FIG. 15 is a perspective view of a composite longitudinal vibrationmechanical filter according to a modification of the mechanical filtershown in FIG. 14;

FIG. 16 is a perspective view of a composite longitudinal vibrationmechanical filter according to still another embodiment of the presentinvention;

FIG. 17 is a perspective view of a composite longitudinal vibrationmechanical filter according to a modification of the mechanical filtershown in FIG. 16;

FIG. 18(a) is a perspective view of a composite longitudinal vibrationmechanical filter in accordance with yet another embodiment of thepresent invention;

FIG. 18(b) is an enlarged fragmentary perspective view of a portion ofthe composite longitudinal vibration mechanical filter shown in FIG.18(a);

FIG. 19 is a perspective view of a composite longitudinal vibrationmechanical filter according to a modification of the mechanical filterillustrated in FIGS. 18(a) and 18(b);

FIG. 20 is a perspective view of a composite longitudinal vibrationmechanical filter according to a still further embodiment of the presentinvention; and

FIG. 21 is a perspective view of a composite longitudinal vibrationmechanical filter according to a modification of the mechanical filtershown in FIG. 20.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A composite longitudinal vibration mechanical filter manufactured by amethod of the present invention will first be described with referenceto FIG. 3. The mechanical filter shown in FIG. 3 has an inputlongitudinally vibratable tuning bar 32 and an output longitudinallyvibratable tuning bar 34 which is identical in shape to the inputlongitudinally vibratable tuning bar 32. The input and outputlongitudinally vibratable tuning bars 32, 34 are disposed in one planeand joined to each other by a pair of thin coupling elements 36, 38 madeof an identity elastic material. Supporting elements 40, 42 projectoutwardly from the centers of the input and output longitudinallyvibratable tuning bars 32, 34. The input and output longitudinallyvibratable tuning bars 32, 34 have respective through grooves 32a, 34adefined longitudinally centrally in and shorter than the longitudinallyvibratable tuning bars 32, 34.

A pair of input piezoelectric ceramic members 44a, 44b is superposed onand fixed to the respective opposite surfaces of the inputlongitudinally vibratable tuning bar 32 by soldering or the like.Likewise, a pair of piezoelectric ceramic members 46a, 46b is superposedon and fixed to the respective opposite surfaces of the outputlongitudinally vibratable tuning bar 34 by soldering or the like.Electrodes (not shown) are metallized or otherwise deposited on thesurfaces of the input piezoelectric ceramic members 44a, 44b and theoutput piezoelectric ceramic members 46a, 46b. The supporting elements40, 42 have outer ends joined to inner opposite edges of a rectangularouter frame 50. The outer frame 50 and the input and outputlongitudinally vibratable tuning bars 32, 34 are disposed in the sameplane. The input and output longitudinally vibratable tuning bars 32,34, the coupling elements 36, 38, the supporting elements 40, 42, andthe outer frame 50 are fabricated as a unitary structure from a singlemetal sheet by etching according to the photolithography which is wellknown in the art as a process for automatically mass-producing ICs withhigh accuracy.

A method of manufacturing the composite longitudinal vibrationmechanical filter shown in FIG. 3 with the photolithographic processwill now be described with reference to FIGS. 2(a) through 2(d).

In a first step shown in FIG. 2(a), a photoresist layer 84 is coated ona flat metal sheet 80. The flat metal sheet 80 is designed such that itcontains, in one plane, the longitudinally vibratable tuning bars 32,34, which have respective grooves 32a, 32b defined centrally in andshorter than the tuning bars 32, 34, the grooves 32a, 32b extending inthe direction in which longitudinal vibration is propagated, thecoupling elements 36, 38, the supporting elements 40, 42, and the outerframe 50, (as seen in FIG. 3) and that it will have desired longitudinalvibration characteristics.

In a second step shown in FIG. 2(b), a radiation such as an X-ray L, forexample, is applied to the flat metal sheet 80 through a mask pattern 86which is of the same shape as the longitudinally vibratable tuning bars32, 34, the grooves 32a, 34a, the coupling elements 36, 38, thesupporting elements 40, 42, and the outer frame 50.

In a third step shown in FIG. 2(c), the flat metal sheet 80 is dipped ina solvent to develop the pattern corresponding to the mask pattern 86,and then photoresist layer areas 87a, 87b, 87c, 87d, and 87e which havebeen developed to the X-ray L are removed from the metal sheet 80.

In a fourth step shown in FIG. 2(d), the portions of the flat metalsheet 80 which correspond to the photoresist layer areas 87a, 87b, 87c,87d, and 87e that have been removed in the third step are removed byetching.

In this manner, the longitudinally vibratable tuning bars 32, 34 withthe grooves 32a, 34a defined centrally therein in the longitudinaldirection thereof and shorter than the tuning bars 32, 34, the couplingelements 36, 38, the supporting elements 40, 32, and the outer frame 50are formed as a unitary structure.

Then, the input and output piezoelectric ceramic members 44a, 44b and46a, 46b on which electrodes of gold or silver are metallized by vacuumevaporation or sputtering, are superposed on and fixed to the input andoutput longitudinally vibratable tuning bars 32, 34 by soldering.

Thereafter, a feed line 52 and a grounding line 52e (FIG. 3) aresoldered to the input piezoelectric ceramic members 44a, 44b and theinput longitudinally vibratable tuning bar 32, and similarly an outletline 54 and a grounding line 54e are soldered to the outputpiezoelectric ceramic members 46a, 46b and the output longitudinallyvibratable tuning bar 34.

Operation of the composite longitudinal vibration mechanical filtershown in FIG. 3, which is manufactured by the above process, will bedescribed below.

A high-frequency signal S₄, such for example as anintermediate-frequency signal having a frequency of 455 KHz generated bya frequency converter in a superheterodyne receiver or the like, issupplied from a signal source Osc to a resistor R and then to the feedline 52 and the grounding line 52e between the input piezoelectricceramic members 44a, 44b and the input longitudinally vibratable tuningbar 32. The applied high-frequency signal S₄ generates an electric fieldhaving the same frequency as that of the signal S₄, between theelectrodes and the input longitudinally vibratable tuning bar 32 whichis electrically grounded. In response to the electric field thusgenerated, the input piezoelectric ceramic members 44a, 44b aremechanically deformed in the directions indicated by the arrows mi, moin FIG. 3, and the input longitudinally vibratable tuning bar 32resonates to produce a longitudinal wave having a frequency F₄ and ahalf wavelength which is equal to the length L₄ of the inputlongitudinally vibratable tuning bar 32. If the longitudinal wave ispropagated along the input longitudinally vibratable tuning bar 32 at anaverage speed V, then the frequency F₄ is given by the followingequation:

    F.sub.4 =V/(2L.sub.4)                                      (3)

The longitudinal vibration of the input longitudinally vibratable tuningbar 32 is mechanically coupled and propagated to the outputlongitudinally vibratable tuning bar 34 to the coupling elements 36, 38,causing the output longitudinally vibratable tuning bar 34 to resonateor vibrate longitudinally at a frequency F₅ and with a half wavelengthequal to the length L₅ of the tuning bar 34. If the longitudinal wave ispropagated along the output longitudinally vibratable tuning bar 34 atan average speed V, then the frequency F₅ is given by the followingequation:

    F.sub.5 =V/(2L.sub.5)                                      (4)

The longitudinal vibration of the output longitudinally vibratabletuning bar 34 produces a voltage between the electrodes on the outputpiezoelectric ceramic members 46a, 46b. The produced voltage is then ledout between the outlet line 54 and the grounding line 54e as an outputsignal S₅, e.g., an intermediate-frequency signal S₅ having a frequencyof 455 KHz, with a sharp frequency characteristic curve created by thetransmission of the longitudinal vibration, i.e., with narrow-bandfrequency characteristics.

As can be understood from the equations (3) and (4) above, the resonantfrequency F₄ of the input longitudinally vibratable tuning bar 32 andthe resonant frequency F₅ of the output longitudinally vibratable tuningbar 34 are inversely proportional to the lengths L₄, L₅ of therespective tuning bars 32, 34. The accuracy of the lengths L₄, L₅ isdependent on the photolithographic technology which is employed tofabricate the longitudinally vibratable tuning bars 32, 34. The accuracyof the lengths L₄, L₅ cannot be achieved with a sufficiently small errorbecause of the thickness of the tuning bars 32, 34. Generally, thedimensional accuracy δL of the length of the tuning bars 32, 34 isexpressed by:

    δL=±1.5/10·t                             (5)

where t is the thickness of the tuning bars 32, 34. The dimensionalaccuracy δL does not vary greatly since the input and outputlongitudinally vibratable tuning bars 32, 34 are simultaneouslyfabricated as a unitary structure by etching.

For a photolithographic process, δL is defined as dimensional accuracyin a horizontal plane which is a function of a vertical dimension. Inother words, the accuracies of the length and width (as applicable) of abar made by a photolithographic process are a function of the thicknessof the bar.

The grooves 32a, 34a defined in the longitudinally vibratable tuningbars 32, 34 of FIG. 3 will hereinafter be described. Since the grooves32a, 34a provide the same advantages for the input and outputlongitudinally vibratable tuning bars 32, 34, one longitudinallyvibratable tuning bar will be described below.

It is assumed that a longitudinally vibratable tuning bar which has awidth w has a central longitudinal groove having a width M and a lengthL_(M), and that the material of which the longitudinally vibratabletuning bar is made has an average mass ρ.

Since the groove defined centrally in FIG. 3 in the longitudinallyvibratable tuning bar extends in the direction in which longitudinalvibration is propagated therealong, the groove does not interfere withoperation of the longitudinally vibratable tuning bar. Thecross-sectional area Sa of the longitudinally vibratable tuning barwhere the groove is present and longitudinal vibration takes place issmall because of the groove. The cross-sectional area Sa is given by:

    Sa=(W-M)·t                                        (6)

The cross-sectional area Sb of the longitudinally vibratable tuning barwhere no groove is present and longitudinal vibration takes place isgiven by:

    Sb=W·t                                            (7)

If the width W of the longitudinally vibratable tuning bar is reduced bythe dimensional accuracy δL due to an etching error (overetching), thenthe groove iS widened by δL. At this time, the cross-sectional areas Sa,Sb are expressed as follows:

    Sa={(W-δL)-(M+δL)}·t=(W-M-2δL)·t(8)

    Sb=(W-δL)·t                                 (9)

The length L of the longitudinally vibratable tuning bar now becomes(L-δL).

The effect of an added mass on the longitudinally vibratable tuning barwill be considered below.

(1) When δL=0, a mass represented by {M·(L-L_(M))·t·ρ} when L_(M) is thelength of the groove and commensurate with the width of thelongitudinally vibratable tuning bar is added to the distal end of thelongitudinally vibratable tuning bar that has the cross-sectional area{(W-M)·t}, and the length of the longitudinally vibratable tuning bar isindicated by L.

(2) When δL≠0, a mass represented by {(M+δL). (L-L_(M))·t·ρ} andcommensurate with the width of the longitudinally vibratable tuning barand a mass represented by {(-δL)·W·t·ρ} and commensurate with the lengthof the longitudinally vibratable tuning bar are added to the distal endof the longitudinally vibratable tuning bar that has the cross-sectionalarea {(W-M-2δL)·t}, and the length of the longitudinally vibratabletuning bar is indicated by L (though the length is indicated by L-δL,the dimensional accuracy δL is considered as an added mass).

As a result of comparison between the equations in the cases (1) and (2)above, the mass δL which is newly added when δL≠0 is given by thefollowing equation:

    δρ=δL·{(L-L.sub.M)-W}·t·ρ.multidot.{(W-M)·t·ρ}/{(W-M-2δL)·t.multidot.ρ}                                                  (10)

If the dimensions of the longitudinally vibratable tuning bar areselected such that (L-L_(M))-W=0, i.e.,

    L.sub.M =L-W                                               (11)

then δρ=0

even when δL≠0.

Therefore, the mass of the longitudinally vibratable tuning bar does notvary, and hence the resonant frequency of the longitudinally vibratabletuning bar does not vary.

As described above, the grooves 32a, 34a which extend in the directionin which longitudinal vibration is propagated are defined centrally inthe input and output longitudinally vibratable tuning bars 32, 34. Evenif the longitudinally vibratable tuning bars 32, 34 have differentlengths due to an etching error which is caused when they are fabricatedas a unitary structure, the central frequency of the compositelongitudinal vibration mechanical filter does not vary and the bandpasscharacteristics thereof are not degraded because of the grooves 32a, 34awhich are defined by etching in the longitudinally vibratable tuningbars 32, 34 at the same time that they are fabricated.

Another composite longitudinal vibration mechanical filter whichincludes five longitudinally vibratable tuning bars and achieves agreater amount of attenuation outside of the passband, i.e., provides asharper frequency characteristic curve, will be described with referenceto FIG. 4.

The composite longitudinal vibration mechanical filter comprises inputand output longitudinally vibratable tuning bars 70, 78, threelongitudinally vibratable tuning bars 72, 74, 76 disposed between thelongitudinally vibratable tuning bars 70, 78, and coupling elements 82a,82b, 84a, 84b, 86a, 86b, 88a, 88b by which the longitudinally vibratabletuning bars 70, 72, 74, 76, 78 are joined together. The longitudinallyvibratable tuning bars 70, 72, 74, 76, 78 have respective longitudinalgrooves 70a, 72a, 74a, 76a, 78a defined centrally therein.

Supporting elements 90, 92 project outwardly centrally from the inputand output longitudinally vibratable tuning bars 70, 78, and have outerends secured to inner opposite edges of an outer frame 93. A pair ofinput piezoelectric ceramic members 94a, 94b is superposed on and fixedto the opposite surfaces of the input longitudinally vibratable tuningbar 70, and a pair of output piezoelectric ceramic members 96a, 96b issuperposed on and fixed to the opposite surfaces of the outputlongitudinally vibratable tuning bar 78.

The composite longitudinal vibration mechanical filter shown in FIG. 4is manufactured and operates in basically the same manner, and offerssubstantially the same advantages, as the composite longitudinalvibration mechanical filter shown in FIG. 3.

However, use of the plural longitudinally vibratable tuning bars 70, 72,74, 76, 78 is effective in greatly reducing dimensional variations ofthese tuning bars, and in improving the passband characteristics of themechanical filter.

In the embodiments shown in FIGS. 3 and 4, the grooves 32a, 34a and 70a,72a, 74a, 76a, 78a are defined through the longitudinally vibratabletuning bars 32, 34 and 70, 72, 74, 76, 78, respectively. However, theinvention is not limited to those grooves. Blind grooves may be definedin these longitudinally vibratable tuning bars, or two or more groovesmay be defined in each of the longitudinally vibratable tuning bars. Agroove or grooves may be defined in one or some of the longitudinallyvibratable tuning bars 32, 34 and 70, 72, 74, 76, 78. A curved ordiscontinuous groove or grooves may be defined in the longitudinallyvibratable tuning bars. The input signal is fed in by leads 122 and 122eand the output signal appears across leads 124 and 124e.

In the manufacturing method shown in FIGS. 2(a) through 2(d), aphotoresist layer which has been exposed to an X-ray is removed, whichis known as the negative process. However, the positive process may beemployed to fabricate the composite longitudinal vibration mechanicalfilter.

As described above, the method of the present invention includes thefirst step of defining grooves in at least the input and outputvibratable bodies along the direction in which the vibratable bodiesvibrate, at the same time that the vibratable bodies are fabricated, thegrooves being shorter than the length of the vibratable bodies, and thesecond step of superposing and fixing the piezoelectric members to theinput and output vibratable bodies in sandwiching relation. The groovesare defined in the vibratable bodies by etching or the like at the sametime that the vibratable bodies are fabricated. The vibratable bodies asthey are mass-produced have a highly accurate central frequency, and areeffectively prevented from dimensionally varying, so that the compositelongitudinal vibration mechanical filter has improved passbandcharacteristics and can well be mass-produced.

A composite longitudinal vibration mechanical filter manufactured by amethod according to another embodiment of the present invention will bedescribed with reference to FIGS. 6(a) and 6(b). The compositelongitudinal vibration mechanical filter illustrated in FIGS. 6(a) and6(b) is of essentially the same configuration as the compositelongitudinal vibration mechanical filter shown in FIG. 3, except thatthe longitudinally vibratable tuning bars have plural through holes132a, 134a defined therein and have opening sizes which are small ascompared with the wavelength of the longitudinal vibration of the tuningbars. The other structural components are the same as those of thecomposite longitudinal vibration mechanical filter shown in FIG. 3.Therefore, those identical components are denoted by identical referencenumerals, and will not be described in detail.

A method of manufacturing the composite longitudinal vibrationmechanical filter shown in FIGS. 6(a) and 6(b) according to thephotolithographic process is illustrated in FIGS. 5(a), 5(b), 5(c) and5(d). The through holes 132a, 134a are defined in the input and outputlongitudinally vibratable tuning bars 32, 34 according to thephotolithographic process at the same time that the longitudinallyvibratable tuning bars 32, 34 are fabricated. Therefore, a mask pattern86 used has through holes 87a, 87b corresponding to the through holes132a, 134a to be defined. In the third step shown in FIG. 5(c),photoresist layer areas 187a, 187b, 187c, 187d, 187e to be removed areformed on the metal sheet 80. The other steps are same as those shown inFIGS. 2(a), 2(b), 2(c) and 2(d). Those identical components are denotedby identical reference numerals, and will not be described in detail. Wis the width of the tuning bar and L_(M) is the length of the grooveand/or the length where the through holes exist.

As can be understood from the equations (3) and (4) above, the resonantfrequency F₄ of the input longitudinally vibratable tuning bar 32 andthe resonant frequency F5 of the output longitudinally vibratable tuningbar 34 are inversely proportional to the lengths L₄, L₅ of therespective tuning bars 32, 34. The accuracy of the lengths L₄, L₅ isdependent on the photolithographic technology which is employed tofabricate the longitudinally vibratable tuning bars 32, 34. The accuracyof the lengths L₄, L₅ cannot be achieved with a sufficiently small errorbecause of the thickness of the tuning bars 32, 34. Generally, thedimensional accuracy δL of the length of the tuning bars 32, 34 isexpressed by:

    δL=±1.5/10·t                             (12)

where t is the thickness of the tuning bars 32, 34. The dimensionalaccuracy δL does not vary greatly since the input and outputlongitudinally vibratable tuning bars 32, 34 with the through holes132a, 134a are simultaneously fabricated as a unitary structure byetching. The signs of the equation (12) remain the same as those of theequation (5).

The through holes 132a, 134a defined in the longitudinally vibratabletuning bars 32, 34 will hereinafter be described.

It is assumed that the longitudinally vibratable tuning bars 32, 34 havea length L (L₄, L₅), the through holes have a width M (see FIG. 6(b)),the distribution ratio of the through holes 132a, 134a in the input andoutput longitudinally vibratable tuning bars 32, 34 (the ratio of thesum of the areas of the through holes to the entire area of the centralportion of the longitudinally vibratable tuning bar) is indicated by γ,the input and output longitudinally vibratable tuning bars 32, 34 have awidth w, and the length of the longitudinally vibratable tuning bars 32,34 where the through holes 132a, 134a are present (in the direction inwhich the longitudinal vibration is propagated) is indicated by L_(M),and also that the material of which the longitudinally vibratable tuningbars 32, 34 are made has an average mass ρ.

Since the through holes 132a, 134a defined in the input and outputlongitudinally vibratable tuning bars 32, 34 are sufficiently small ascompared with the wavelength of the longitudinal vibration, the throughholes 132a, 134a do not interfere with operation of the input and outputlongitudinally vibratable tuning bars. The cross-sectional area S_(c) ofthe longitudinally vibratable tuning bars where the through holes 132a,134a are present and longitudinal vibration takes place is small becauseof the through holes 132a, 134a. The cross-sectional area S_(c) is givenby:

    S.sub.c =(W-γM)·t                           (13)

The cross-sectional area S_(d) of the longitudinally vibratable tuningbars where no through holes are present and longitudinal vibration takesplace is given by:

    S.sub.d =W·t                                      (14)

If the width W of the longitudinally vibratable tuning bars is reducedby the dimensional accuracy δL due to an etching error (overetching),then each of the through holes 132a, 134a is widened by δL. At thistime, the cross-sectional areas S_(c), S_(d) are expressed as follows:##EQU1##

The length L (L₄, L₅) of the input and output longitudinally vibratabletuning bars 32, 34 now becomes (L-δL).

The effect of an added mass on the input and output longitudinallyvibratable tuning bars 32, 34 will be considered below.

(1) When δL=0, a mass represented by {γM·(L-L_(M))·t·ρ} and commensuratewith the width W of the longitudinally vibratable tuning bars is addedto the distal ends of the input and output longitudinally vibratabletuning bars 32, 34 that have the cross-sectional area {(W-γM)·t}, andthe length of the input and output longitudinally vibratable tuning bars32, 34 is indicated by L.

(2) When δL≠0, a mass represented by {(γM+γδL). (L-L_(M))·t·ρ} andcommensurate with the width W of the longitudinally vibratable tuningbars and a mass represented by {(-δL)·W·t·ρ} and commensurate with thelength L of the longitudinally vibratable tuning bars are added to thedistal ends of the input and output longitudinally vibratable tuningbars 32, 34 that have the cross-sectional area {(W-γM-(1+γ) δL)·t}, andthe length of the longitudinally vibratable tuning bars is indicated byL (though the length is indicated by L-δL, the dimensional accuracy δLis considered as an added mass).

As a result of comparison between the equations in the cases (1) and (2)above, the mass δρ which is newly added when δL≠0 is given by thefollowing equation:

    δρ=δL·{γ(L-L.sub.M)-W}·t·.rho.·{(W-γM)·t·ρ}/{(W-γM-(1+.gamma.) δL)·t·ρ}                  (17)

If the dimensions of the longitudinally vibratable tuning bars areselected such that γ(L-L_(M))-W=0, i.e.,

    L.sub.M =L-W/γ                                       (18)

then δρ=0

even when δL≠0.

Therefore, the mass of the longitudinally vibratable tuning bars doesnot vary, and hence the resonant frequency of the input and outputlongitudinally vibratable tuning bars does not vary.

As described above, the through holes 132a, 134a which have an openingsize sufficiently smaller than the wavelength of the longitudinalvibration is defined in the input and output longitudinally vibratabletuning bars 32, 34. Even if the longitudinally vibratable tuning bars32, 34 do not have a sufficient dimensional accuracy, i.e., they havedifferent lengths due to an etching error which is caused when they arefabricated as a unitary structure, the central frequency of thecomposite longitudinal vibration mechanical filter does not vary and thebandpass characteristics thereof are not degraded because of the throughholes 32a, 34a which are defined by etching in the longitudinallyvibratable tuning bars 32, 34 at the same time that they are fabricated.

Another composite longitudinal vibration mechanical filter whichincludes five longitudinally vibratable tuning bars and achieves agreater amount of attenuation outside of the passband, i.e., provides asharper frequency characteristic curve, will be described with referenceto FIG. 7.

The composite longitudinal vibration mechanical filter shown in FIG. 7is substantially identical in construction to the composite longitudinalvibration mechanical filter shown in FIG. 4, except that thelongitudinally vibratable tuning bars have plural through holes 170a,172a, 174a, 176a, 178a defined therein and having opening sizes whichare small as compared with the wavelength of the longitudinal vibrationof the tuning bars. The input signal is fed in via leads 122 and 122eand the output signal appears across leads 124 and 124e. The otherstructural components are the same as those of the compositelongitudinal vibration mechanical filter shown in FIG. 4. Therefore,those identical components are denoted by identical reference numerals,and will not be described in detail.

The composite longitudinal vibration mechanical filter shown in FIG. 7is manufactured by the same photolithographic process and operates inthe same manner as the composite longitudinal vibration mechanicalfilter shown in FIG. 6.

However, use of the plural longitudinally vibratable tuning bars 70, 72,74, 76, 78 is effective in greatly reducing dimensional variations ofthese tuning bars, and in improving the passband characteristics of themechanical filter.

In the embodiments shown in FIGS. 6 and 7, the holes 32a, 34a and 170a,172a, 174a, 176a, 178a are defined through the longitudinally vibratabletuning bars 32, 34 and 70, 72, 74, 76, 78, respectively. However, theinvention is not limited to those through holes. Through holes and/orblind holes or recesses may be defined in these longitudinallyvibratable tuning bars, or two or more holes may be defined in one orsome of the longitudinally vibratable tuning bars 32, 34, and 70, 72,74, 76, 78.

As described above, the embodiments shown in FIGS. 6 and 7 reside inthat through holes and/or blind holes may be defined in at least one ofthe vibratable bodies including the input and output vibratable bodies.

with the above arrangement, the composite longitudinal vibrationmechanical filter has a highly accurate central frequency, improvedpassband characteristics, reduced characteristic variations between thelongitudinally vibratable tuning bars, provides uniform characteristicswhen it is mass-produced, and is of improved quality.

FIG. 9 shows a composite longitudinal vibration mechanical filter whichis manufactured by a method according to still another embodiment of thepresent invention. The composite longitudinal vibration mechanicalfilter has coupling elements 36, 38 which are positioned near the distalends of the input and output longitudinally vibratable tuning bars 32,34, i.e., in regions where the longitudinally vibratable tuning bars aredisplaced to a large extent in the direction in which the longitudinalvibration takes place. The vibration is propagated (coupled) to thecoupling elements 36, 38 as a transverse wave, i.e., so-called flexuralvibration, so that spurious responses are reduced and the passbandcharacteristics are improved.

The composite longitudinal vibration mechanical filter shown in FIG. 9is substantially identical to the composite longitudinal vibrationmechanical filter shown in FIG. 3, except that grooves 32a, 32a are notdefined. Those parts in FIG. 9 which are identical to those of FIG. 3are designated by identical reference numerals, and will not bedescribed in detail.

The composite longitudinal vibration mechanical filter shown in FIG. 9is manufactured by the photolithographic process in substantially thesame manner as shown in FIGS. 2(a), 2(b), 2(c) and 2(d).

Reduction of spurious responses with the structure shown in FIG. 9 willbe described below.

The input and output longitudinally vibratable tuning bars 32, 34 aredisplaced to a greater extent at their distal ends in the direction inwhich the longitudinal vibration takes place. Displacement of thelongitudinally vibratable tuning bars 32, 34 in a direction normal tothe longitudinal direction is greater at the center of thelongitudinally vibratable tuning bars 32, 34. The displacement by thelongitudinal vibration of the input longitudinally vibratable tuning bar32 in the direction of the longitudinal vibration, and the displacementthereof in the direction normal to the longitudinal vibration, aretransmitted (coupled) to the output longitudinally vibratable tuning bar34 via the coupling elements 36, 38.

At this time, not only the displacement normal to the longitudinalvibration is coupled to the longitudinal vibration of the outputlongitudinally vibratable tuning bar 34 via the coupling elements 36,38, but also vibration in another mode is coupled to the longitudinalvibration of the output longitudinally vibratable tuning bar 34.Therefore, spurious responses are of a large value, deteriorating thefilter characteristics. The vibration normal to the longitudinalvibration is propagated mainly as a longitudinal wave in the couplingelements 36, 38, and the displacement in the direction of thelongitudinal vibration produces smaller spurious responses than thedisplacement normal to the longitudinal vibration as it is coupled tothe longitudinal vibration of the output longitudinally vibratabletuning bar 34 via the coupling elements 36, 38. The longitudinalvibration is propagated as flexural vibration in the coupling elements36, 38.

The coupling elements 36, 38 are disposed in the regions where thedisplacement in the direction of the longitudinal vibration is large,near the distal ends of the input and output longitudinally vibratabletuning bars 32, 34. The input and output longitudinally vibratabletuning bars 32, 34 are coupled to each other by the flexural vibrationvia the coupling elements 36, 38. Accordingly, spurious responses arereduced, and the passband characteristics are improved.

The displacement of the input longitudinally vibratable tuning bar 32 inthe direction of the longitudinal vibration is larger at its distalends, and is represented as a function of the position in the directionof the longitudinal vibration. In order to provide desired frequencycharacteristics and reduce dimensional variations of the longitudinallyvibratable tuning bars, it is necessary to uniformize the amount ofcoupling of the output longitudinally vibratable tuning bar 34 to theinput longitudinally vibratable tuning bar 32. The coupling elements 36,38 should be positioned relatively to the input longitudinallyvibratable tuning bar 32 as constantly as possible. More specifically,the relative position between the input and output longitudinallyvibratable tuning bars 32, 34 and the coupling elements 36, 38 can berendered constant by fabricating the input and output longitudinallyvibratable tuning bars 32, 34 and the coupling elements 36, 38 from asingle sheet by etching according to the photolithographic process.FIGS. 8(a) through 8(d) show a method of manufacturing the compositelongitudinal vibration mechanical filter shown in FIG. 9, the methodbeing essentially the same as the method shown in FIGS. 2(a) through2(d).

Another composite longitudinal vibration mechanical filter whichcomprises five longitudinally vibratable tuning bars and providesincreased frequency attenuation outside of the passband is illustratedin FIG. 10.

The composite longitudinal vibration mechanical filter shown in FIG. 10comprises input and output longitudinally vibratable tuning bars 270,278, three longitudinally vibratable tuning bars 272, 274, 276 disposedbetween the longitudinally vibratable tuning bars 270, 278, and couplingelements 282a, 282b, 284a, 284b, 286a, 286b, 288a, 288b by which thelongitudinally vibratable tuning bars 270, 272, 274, 276, 278 are joinedtogether.

Supporting elements 290, 292 project outwardly centrally from the inputand output longitudinally vibratable tuning bars 270, 278, and haveouter ends secured to inner opposite edges of an outer frame 293. A pairof input piezoelectric ceramic members 294a, 294b is superposed on andfixed to the opposite surfaces of the input longitudinally vibratabletuning bar 270, and a pair of output piezoelectric ceramic members 296a,296b is superposed on and fixed to the opposite surfaces of the outputlongitudinally vibratable tuning bar 278. The composite longitudinalvibration mechanical filter also has a feed line 297 and a groundingline 297e which are connected respectively to the input piezoelectricceramic members 294a, 294b, and an outlet line 298 and a grounding line298e which are connected respectively to the output piezoelectricceramic members 296a, 296b.

The composite longitudinal vibration mechanical filter shown in FIG. 10is manufactured and operates in basically the same manner as thecomposite longitudinal vibration mechanical filter shown in FIG. 9.

With the plural longitudinally vibratable tuning bars 270, 272, 274,276, 278 are employed, dimensional variations of these tuning bars arereduced, and the passband characteristics of the mechanical filter areimproved.

According to the above embodiments shown in FIGS. 9 and 10, thecomposite longitudinal vibration mechanical filter for delivering asupplied high-frequency signal in a predetermined frequency rangeincludes a plurality of longitudinally vibratable bodies including inputand output vibratable bodies for receiving and delivering thehigh-frequency signal, the vibratable bodies being vibratable in a rangeclose to the passband of the mechanical filter, a plurality ofpiezoelectric members superposed on the input and output vibratablebodies and including electrodes to which conductors are connected, aplurality of coupling elements disposed between ends of the vibratablebodies and coupling them through flexural vibration, a plurality ofsupporting members projecting from the input and output vibratablebodies, and a holder member which holds the vibratable bodies includingthe input and output vibratable bodies with the supporting membersprojecting therefrom.

With such an arrangement, the composite longitudinal vibrationmechanical filter has a highly accurate central frequency, improvedpassband characteristics, provides uniform characteristics when it ismass-produced, and is of improved quality.

FIG. 11 shows a composite longitudinal vibration mechanical filteraccording to another embodiment of the present invention. The compositelongitudinal vibration mechanical filter shown in FIG. 11 comprises aninput longitudinally vibratable tuning bar 332 and an outputlongitudinally vibratable tuning bar 334 which is identical in shape tothe tuning bar 332. The input and output longitudinally vibratabletuning bars 332, 334 lie in one plane, and joined to each other bycoupling elements 336, 338 which are made of an identity elasticmaterial. Supporting elements 340, 341 project outwardly from thecenters of the input and output longitudinally vibratable tuning bars332, 334. The supporting elements 340, 341 each have a portion to whichthere are connected respective vibration absorbing body holders 342, 343with vibration absorbing bodies 342a, 343a secured respectively thereto.The vibration absorbing bodies 342a, 343a, which are made of aviscoelastic material such as silicone rubber, for example, convertstransmitted vibration to Joule heat.

A pair of input piezoelectric ceramic members 344a, 344b is superposedon and fixed to the respective opposite surfaces of the inputlongitudinally vibratable tuning bar 332 by soldering or the like.Likewise, a pair of output piezoelectric ceramic members 346a, 346b issuperposed on and fixed to the respective opposite surfaces of theoutput longitudinally vibratable tuning bar 334 by soldering or thelike. Electrodes (not shown) are metallized or otherwise deposited onthe surfaces of the input piezoelectric ceramic members 344a, 344b andthe output piezoelectric ceramic members 346a, 346b.

The supporting elements 340, 341 have outer ends, supporting elements348 and 349, respectively, joined to inner opposite edges of arectangular outer frame 350. The outer frame 350 and the input andoutput longitudinally vibratable tuning bars 332, 334 are disposed inthe same plane. The input and output longitudinally vibratable tuningbars 332, 334, the coupling elements 336, 338, the supporting elements340, 342, and the outer frame 350 are fabricated as a unitary structurefrom a single metal sheet by etching according to the photolithography,for example.

A feed line 352 and a grounding line 352e for supplying a high-frequencysignal are connected to the input piezoelectric ceramic members 344a,344b, respectively, and an outlet line 354 and a grounding line 354e fordelivering out an output signal are connected to the outputpiezoelectric ceramic members 346a, 346b, respectively.

The composite longitudinal vibration mechanical filter of the aboveconstruction operates as follows: When a high-frequency signal isapplied to the input piezoelectric ceramic members 344a, 344b, the inputlongitudinally vibratable tuning bar 332 is longitudinally vibrated inthe directions indicated by the arrows mi, mo. Such longitudinalvibration is transmitted through the coupling elements 336, 338 to theoutput longitudinally vibratable tuning bar 334. The frequencies of thelongitudinal vibration of the input and output longitudinally vibratabletuning bars 332, 334 are expressed by the previously described equations(3) and (4). At the same time, the input longitudinally vibratabletuning bar 332 is also vibrated in directions normal to the directionsindicated by the arrows mi, mo, i.e., in the axial or longitudinaldirection of the supporting element 340. This vibration is propagated inthe supporting element 340 and absorbed by the vibration absorbing body342a fixedly mounted on the vibration absorbing body holder 342. Anyvibration which has not been absorbed by the vibration absorbing body342a is propagated through the outer frame 350 and then the supportingelement 349 joined to the output longitudinally vibratable tuning bar334. Such propagated vibration is then absorbed by the vibrationabsorbing body 343a mounted on the vibration absorbing body holder 343to which the supporting element 349 is fixed. As a result, the unwantedvibration is minimized before it is transmitted to the outputlongitudinally vibratable tuning bar 334.

The undesired vibratory wave emitted from the input longitudinallyvibratable tuning bar 332 toward the output longitudinally vibratabletuning bar 334 is effectively absorbed by the vibration absorbing bodies342a, 343a mounted respectively on the vibration absorbing body holders342, 343, so that unwanted spurious responses outside of the passbandwill be reduced.

Still another composite longitudinal vibration mechanical filter whichincludes five longitudinally vibratable tuning bars and achieves agreater amount of attenuation outside of the passband will be describedwith reference to FIG. 12.

The composite longitudinal vibration mechanical filter illustrated inFIG. 12 comprises input and output longitudinally vibratable tuning bars370, 378, three longitudinally vibratable tuning bars 372, 374, 376disposed between the longitudinally vibratable tuning bars 370, 378, andcoupling elements 382a, 382b, 384a, 384b, 386a, 386b, 388a, 388b bywhich the longitudinally vibratable tuning bars 370, 372, 374, 376, 378are joined together.

Supporting elements 390, 392 project outwardly centrally from the inputand output longitudinally vibratable tuning bars 370, 378, and haveouter ends secured to respective vibration absorbing body holders 396,398 on which respective vibration absorbing bodies 396a, 398a arefixedly mounted. A pair of input piezoelectric ceramic members 399a,399b is superposed on and fixed to the opposite surfaces of the inputlongitudinally vibratable tuning bar 370, and a pair of outputpiezoelectric ceramic members 387a, 387b is superposed on and fixed tothe opposite surfaces of the output longitudinally vibratable tuning bar378. Supporting elements 381, 383 which project outwardly from thevibration absorbing body holders 396, 398 have outer ends attached toinner confronting edges of a rectangular outer frame 385. A feed line375 and a grounding line 375e are connected respectively to the inputpiezoelectric ceramic members 399a, 399b, and an outlet line 377 and agrounding line 377e are connected respectively to the outputpiezoelectric ceramic members 387a, 387b.

The composite longitudinal vibration mechanical filter shown in FIG. 12operates in basically the same manner as the composite longitudinalvibration mechanical filter shown in FIG. 11.

However, use of the plural longitudinally vibratable tuning bars 370,372, 374, 376, 378 is effective in increasing the amount of attenuationoutside of the passband of the mechanical filter, and the vibrationabsorbing bodies 396a, 398a absorb undesired vibratory waves, thussuppressing spurious responses to a greater extent.

FIG. 13 shows the passband characteristics of the mechanical filtershown in FIG. 11. A study of FIG. 13 indicates that the unwantedvibration is effectively absorbed by the vibration absorbing bodies342a, 343a, so that undesired spurious responses outside of the passbandare reduced. In FIG. 13, the amount of attenuation outside of thepassband is 40 dB without any vibration absorbing bodies, but isincreased to 65 dB with the vibration absorbing bodies 342a, 343aemployed.

In the embodiments shown in FIGS. 11 and 12, the vibration absorbingbody holders 342, 343, 396, 398 and the vibration absorbing bodies 342a,343a, 396a, 398a are rectangular in shape when viewed in plan, and thevibration absorbing bodies 342a, 343a, 396a, 398a are mounted on theupper surfaces of the vibration absorbing body holders 342, 343, 396,398. However, the present invention is not limited to the illustratedstructures. The vibration absorbing bodies 342a, 343a, 396a, 398a may bedisposed on both surfaces of the vibration absorbing body holders 342,343, 396, 398. The vibration absorbing body holders may be of a circularor rod shape, or may be of a hollow structure for holding many vibrationabsorbing bodies therein, or may be of a combination of theseconfigurations.

As described above, the composite longitudinal vibration mechanicalfilters shown in FIGS. 11 and 12 include vibration absorbing bodyholders disposed between the opposite ends of supporting elements, andvibration absorbing bodies fixedly mounted on the vibration absorbingbody holders, respectively. Unwanted vibratory waves transmitted fromthe input longitudinally vibratable tuning bar through the supportingelements and the holders toward the output longitudinally vibratabletuning bar are effectively absorbed and suppressed by the vibrationabsorbing bodies, thereby reducing undesired spurious responses outsideof the passband of the mechanical filter and improving the passbandcharacteristics thereof.

FIG. 14 shows a composite longitudinal vibration mechanical filteraccording to a further embodiment of the present invention. Thecomposite longitudinal vibration mechanical filter shown in FIG. 14comprises an input longitudinally vibratable tuning bar 432 and anoutput longitudinally vibratable tuning bar 434 which is identical inshape to the tuning bar 432. The input and output longitudinallyvibratable tuning bars 432, 434 lie in one plane, and joined to eachother by narrow coupling elements 436, 438 which are made of an identityelastic material. The input and output longitudinally vibratable tuningbars 432, 434 have resonant frequency adjusting fingers 432a, 432b,432c, 432d and 434a, 434b, 434c, 434d on their distal ends. The otherstructural details of the composite longitudinal vibration mechanicalfilter shown in FIG. 14 are the same as those of the mechanical filtershown in FIG. 9. Those parts shown in FIG. 14 which are identical tothose shown in FIG. 9 are denoted by identical reference numerals, andwill not be described.

Operation of the composite longitudinal vibration mechanical filterillustrated in FIG. 14 is as follows:

As can be understood from the equations (3) and (4) referred topreviously, the resonant frequency F₄ of the input longitudinallyvibratable tuning bar 432 and the resonant frequency F₅ of the outputlongitudinally vibratable tuning bar 434 are determined by the lengthsL₄, L₅ of the respective tuning bars 432, 434. The accuracy of thelengths L₄, L₅ is dependent on the photolithographic technology which isemployed to fabricate the longitudinally vibratable tuning bars 432,434. The accuracy of the lengths L₄, L₅ cannot be achieved withsufficiently small error because of the thickness of the tuning bars432, 434. Generally, the dimensional accuracy δL of the length of thetuning bars 432, 434 is expressed by:

    δL=±1.5/10·t                             (19)

where t is the thickness of the tuning bars 332, 334. Therefore, theaccuracies of resonant frequencies F₄, F₅ of the input and outputlongitudinally vibratable tuning bars 432, 434 cannot be higher than theaccuracies which are expressed by the following equations:

    δL.sub.4 =±1.5/10·t/L.sub.4              (20)

    δL.sub.5 =±1.5/10·t/L.sub.5              (21)

The thickness t is generally selected to be in the range of from 0.01 Lto 0.1 L. Consequently, the relative accuracy of the frequency given bythe equations (20) and (21) ranges from ±0.0015·F to ±0.015·F (Frepresents the central frequency of the mechanical filter). Thisfrequency accuracy is however not sufficient for anintermediate-frequency filter for use in communication devices.

The resonant frequency adjusting fingers 432a, 432b, 432c, and 432d onthe ends of the input longitudinally vibratable tuning bar 432 and theresonant frequency adjusting fingers 434a, 434b, 434c, 434d on theoutput longitudinally vibratable tuning bar 434 operate as follows:

Each of the resonant frequency adjusting fingers 432a, 432b, 432c and432d and 434a, 434b, 434c and 434d has a width ω which is smaller thanthe width w of each of the input and output longitudinally vibratabletuning bars 432, 434. The narrower resonant frequency adjusting fingers432a, 432b, 432c and 434d and 434a, 434b, 434c and 434d do not serve aspropagation paths for the longitudinal vibration propagated in the inputand output longitudinally vibratable tuning bars 432, 434, but as addedmasses attached to the propagation paths which are provided by the inputand output longitudinally vibratable tuning bars 432, 434. The addedmasses act to reduce the resonant frequencies of the input and outputlongitudinally vibratable tuning bars 432, 434, and the reduction has amagnitude of approximately ω/W, which is relatively small as compared tothe case where the frequency reduction is achieved by an addition to thepropagation path. The resonant frequencies of the input and outputlongitudinally vibratable tuning bars 432, 434 can be easily and finelyadjusted by altering the dimensions of the resonant frequency adjustingfingers 432a, 432b, 432c and 432d and 434a, 434b, 434c and 434d.

The frequency adjustment may be carried out as follows: The compositelongitudinal vibration mechanical filter is operated, and its passbandcharacteristics and reflecting characteristics are measured. Deviationsof the resonant frequencies F₄, F₅ of the input and outputlongitudinally vibratable tuning bars 432, 434 are then estimated fromthe measured values. The dimensions, such as the length, thickness, andwidth, of one or more of the resonant frequency adjusting fingers 432a,432b, 432c and 432d and 434a 434b, 434c and 434d are reduced inproportion to the estimated frequency deviations. The resonant frequencyadjusting fingers 432a, 432b, 432c and 432d and 434a, 434b, 434c and434d should be of such a large size, in advance, that they can bereduced in dimensions for frequency adjustment.

A modified composite longitudinal vibration mechanical filter whichincludes five longitudinally-vibratable tuning bars and achieves agreater amount of attenuation outside of the passband is shown FIG. 15.

The composite longitudinal vibration mechanical filter illustrated inFIG. 15 comprises input and output longitudinally vibratable tuning bars470, 478, three longitudinally vibratable tuning bars 472, 474, 476disposed between the longitudinally vibratable tuning bars 470, 478, andcoupling elements 482a, 482b, 484a, 484b, 486a, 486b, 488a, 488b bywhich the longitudinally vibratable tuning bars 470, 472, 474, 476, 478are joined together. The longitudinally vibratable tuning bars 470, 472,474, 476, 478 have resonant frequency adjusting fingers 470a, 470b, 470cand 470d, 472a, 472b, 472c and 472d, 474a, 474b, 474c and 474d, 476a,476b, 476c and 476d, and 478a, 478b, 478c and 478d, respectively, ontheir distal ends.

Supporting elements 490, 492 project outwardly centrally from the inputand output longitudinally vibratable tuning bars 470, 478, and haveouter ends secured to respective inner confronting edges of an outerframe 497. A pair oflinput piezoelectric ceramic members 499a, 499b issuperposed on and fixed to the opposite surfaces of the inputlongitudinally vibratable tuning bar 470, and a pair of outputpiezoelectric ceramic members 495a, 495b is superposed on and fixed tothe opposite surfaces of the output longitudinally vibratable tuning bar478. A feed line 491 and a grounding line 491e are connectedrespectively to the input piezoelectric ceramic members 499a, 499b, andan outlet line 493 and a grounding line 493e are connected respectivelyto the output piezoelectric ceramic members 495a, 495b.

The composite longitudinal vibration mechanical filter shown in FIG. 15operates in basically the same manner as the composite longitudinalvibration mechanical filter shown in FIG. 14.

The plural longitudinally vibratable tuning bars 470, 472, 474, 476, 478allow their respective resonant frequencies to be independently adjustedby the resonant frequency adjusting fingers 470a, 470b, 470c and 470d,472a, 472b, 472c and 472d, 474a, 474b, 474c and 474d, 476a, 476b, 476cand 476d, and 478a, 478b, 478c and 478d. Therefore, the resonantfrequencies can be adjusted to desired values for improved passbandcharacteristics.

The resonant frequencies can quickly be adjusted by reducing thedimensions of one or more of the resonant frequency adjusting fingers470a, 470b, 470c and 470d, 472a, 472b, 472c and 472d, 474a, 474b, 474cand 474d, 476a, 476b, 476c and 476d, and 478a, 478b, 478c and 478d witha laser beam in a non-contact manner while measuring the passbandcharacteristics or reflecting characteristics of the compositelongitudinal vibration mechanical filter.

In the embodiments shown in FIGS. 14 and 15, the resonant frequencyadjusting fingers 432a, 432b, 432c and 432d; 434a, 434b, 434c and 434d;470a, 470b, 470c and 470d; 472a, 472b, 472c and 472d; 474a, 474b, 474cand 474d; 476a, 476b, 476c and 476d; and 478a, 478b, 478c and 478d arerespectively disposed on the longitudinal ends of the longitudinallyvibratable tuning bars 430, 434 and 470, 472, 474, 476, 478. However,the resonant frequency adjusting fingers may be disposed on transversesides or upper or lower surfaces of the longitudinally vibratable tuningbars, and may also be dimensionally reduced for frequency adjustment. InFIGS. 14 and 15, each tuning bar 432, 434, 470, 472, 476 and 478 hasfour resonant frequency adjusting fingers designated a, b, c and d,respectively.

Instead of dimensionally reducing the resonant frequency adjustingfingers for frequency adjustment, any other suitable material such as asoldering material may be added to reduce the resonant frequencies ofthe longitudinally vibratable tuning bars.

As described above, the composite longitudinal vibration mechanicalfilters shown in FIGS. 14 and 15 have at least one resonant frequencyadjusting finger disposed on a plurality of vibratable bodies includinginput and output vibratable bodies. The composite longitudinal vibrationmechanical filters have a highly accurate central frequency. After thepassband characteristics of the mechanical filters have been measured,the dimensions such as the length of the resonant frequency adjustingfingers can easily be altered for resonant frequency adjustment so thatvariations of the characteristics of the longitudinally vibratabletuning bars can be reduced. Through such adjustment, compositelongitudinal vibration mechanical filters which are mass-produced canhave uniformized central frequencies and passband characteristics, andhence have an improved quality.

FIG. 16 shows a composite longitudinal vibration mechanical filteraccording to still another embodiment of the present invention.

The composite longitudinal vibration mechanical filter shown in FIG. 16has an input longitudinally vibratable tuning bar 532 and an outputlongitudinally vibratable tuning bar 534 which is identical in shape tothe input longitudinally vibratable tuning bar 532. The input and outputlongitudinally vibratable tuning bars 532, 534 are disposed in one planeand joined to each other by a pair of thin coupling elements 536, 538made of an identity elastic material. Supporting elements 540, 542project outwardly from the centers of the input and outputlongitudinally vibratable tuning bars 532, 534. The input and outputlongitudinally vibratable tuning bars 532, 534 have respective throughgrooves 532a, 534a defined longitudinally centrally in and shorter thanthe longitudinally vibratable tuning bars 532, 534. The other structuraldetails of the mechanical filter of FIG. 16 are the same as those of themechanical filter of FIG. 3. Those other components shown in FIG. 16which are identical to those shown in FIG. 3 are designated by identicalreference numerals, and will not be described in detail.

Operation of the composite longitudinal vibration mechanical filtershown in FIG. 16 will be described below.

As can be understood from the equations (3) and (4) above, the resonantfrequency F₄ of the input longitudinally vibratable tuning bar 532 andthe resonant frequency F₅ of the output longitudinally vibratable tuningbar 534 are inversely proportional to the lengths L₄, L₅ of therespective tuning bars 532, 534. The accuracy of the lengths L₄, L₅ isdependent on the photolithographic technology which is employed tofabricate the longitudinally vibratable tuning bars 532, 534. Theaccuracy of the lengths L₄, L₅ cannot be achieved with sufficientlysmall error because of the thickness of the tuning bars 532, 534.Generally, the dimensional accuracy δL of the length of the tuning bars532, 534 is expressed by:

    δL=±1.5/10·t                             (22)

where t is the thickness of the tuning bars 532, 534. The dimensionalaccuracy δL does not vary greatly since the input and outputlongitudinally vibratable tuning bars 532, 534 are simultaneouslyfabricated as a unitary structure by etching or the like.

The grooves 532a, 534a defined in the longitudinally vibratable tuningbars 532, 534 will hereinafter be described. Since the grooves 532a,534a provide the same advantages for the input and output longitudinallyvibratable tuning bars 532, 534, one longitudinally vibratable tuningbar will be described below.

It is assumed that a longitudinally vibratable tuning bar which has awidth w has a central longitudinal groove having a width M and a lengthL_(M), and that the material of which the longitudinally vibratabletuning bar is made has an average mass ρ.

Since the groove defined centrally in the longitudinally vibratabletuning bar extends in the direction in which longitudinal vibration ispropagated therethrough, the groove does not interfere with operation ofthe longitudinally vibratable tuning bar. The cross-sectional area Sa ofthe longitudinally vibratable tuning bar where the groove is present andlongitudinal vibration takes place is small because of the groove. Thecross-sectional area Sa is given by:

    Sa=(W-M)·t                                        (23)

The cross-sectional area Sb of the longitudinally vibratable tuning barwhere no groove is present and longitudinal vibration takes place isgiven by:

    Sb=W·t                                            (24)

If the width W of the longitudinally vibratable tuning bar is reduced bythe dimensional accuracy δL due to an etching error (overetching), thenthe groove is widened by δL. At this time, the cross-sectional areas Sa,Sb are expressed as follows: ##EQU2##

The length L of the longitudinally vibratable tuning bar now becomes(L-δL).

The effect of an added mass on the longitudinally vibratable tuning barwill be considered below.

(1) When δL=0, a mass represented by {M·(L-L_(M))·t·ρ} and commensuratewith the width of the longitudinally vibratable tuning bar is added tothe distal end of the longitudinally vibratable tuning bar that has thecross-sectional area {(W-M)·t}, and the length of the longitudinallyvibratable tuning bar is indicated by L.

(2) When δL≠0, a mass represented by {(M+δL). (L-L_(M))·t·ρ} andcommensurate with the width of the longitudinally vibratable tuning barand a mass represented by {(-δL)·W·t·ρ} and commensurate with the lengthof the longitudinally vibratable tuning bar are added to the distal endof the longitudinally vibratable tuning bar that has the cross-sectionalarea {(W-M-2δL)·t}, and the length of the longitudinally vibratabletuning bar is indicated by L (though the length is indicated by L-δL,the dimensional accuracy δL is considered as an added mass).

As a result of comparison between the equations in the cases (1) and (2)above, the mass δρ which is newly added when δL≠0 is given by thefollowing equation:

    δρ=δL·{(L-L.sub.M)-W}·t·ρ.multidot.{(W-M)·t·ρ}/{(W-M-2δL)·t.multidot.ρ}                                                  (27)

If the dimensions of the longitudinally vibratable tuning bar areselected such that (L-L_(M))-W=0, i.e.,

    L.sub.M =L-W                                               (28)

then δρ=0

even when δL≠0.

Therefore, the mass of the longitudinally vibratable tuning bar does notvary, and hence the resonant frequency of the longitudinally vibratabletuning bar does not vary.

As described above, the grooves 532a, 534a which extend in the directionin which longitudinal vibration is propagated are defined centrally inthe input and output longitudinally vibratable tuning bars 532, 534.Even if the longitudinally vibratable tuning bars 532, 534 havedifferent lengths due to an etching error which is caused when they arefabricated as a unitary structure, the central frequency of thecomposite longitudinal vibration mechanical filter does not vary and thebandpass characteristics thereof are not degraded because of the grooves532a, 534a which are defined by etching in the longitudinally vibratabletuning bars 532, 534 at the same time that they are fabricated.

Another composite longitudinal vibration mechanical filter whichincludes five longitudinally vibratable tuning bars and achieves agreater amount of attenuation outside of the passband will be describedwith reference to FIG. 17.

The composite longitudinal vibration mechanical filter comprises inputand output longitudinally vibratable tuning bars 570, 578, threelongitudinally vibratable tuning bars 572, 574, 576 disposed between thelongitudinally vibratable tuning bars 570, 578, and coupling elements582a, 582b, 584a, 584b, 586a, 586b, 588a, 588b by which thelongitudinally vibratable tuning bars 570, 572, 574, 576, 578 are joinedtogether. The longitudinally vibratable tuning bars 570, 572, 574, 576,578 have respective longitudinal grooves 570a, 572a, 574a, 576a, 578adefined centrally therein.

Supporting elements 590, 592 project outwardly centrally from the inputand output longitudinally vibratable tuning bars 570, 578, and haveouter ends secured to inner opposite edges of an outer frame 597. A pairof input piezoelectric ceramic members 599a, 599b is superposed on andfixed to the opposite surfaces of the input longitudinally vibratabletuning bar 570, and a pair of output piezoelectric ceramic members 595a,595b is superposed on and fixed to the opposite surfaces of the outputlongitudinally vibratable tuning bar 578. A feed line 591 and agrounding line 591e are connected respectively to the inputpiezoelectric ceramic members 599a, 599b, and an outlet line 593 and agrounding line 593e are connected respectively to the outputpiezoelectric ceramic members 595a, 595b.

The composite longitudinal vibration mechanical filter shown in FIG. 17operates in basically the same manner as the composite longitudinalvibration mechanical filter shown in FIG. 16.

However, use of the plural longitudinally vibratable tuning bars 570,572, 574, 576, 578 is effective in greatly reducing dimensionalvariations of these tuning bars, and in improving the passbandcharacteristics of the mechanical filter.

In the embodiments shown in FIGS. 16 and 17, the grooves 532a, 534a and570a, 572a, 574a, 576a, 578a are defined through the longitudinallyvibratable tuning bars 532, 534 and 570, 572, 574, 576, 578,respectively. However, the invention is not limited to those grooves.Blind grooves may be defined in these longitudinally vibratable tuningbars, or two or more grooves may be defined in each of thelongitudinally vibratable tuning bars. A groove or grooves may bedefined in one or some of the longitudinally vibratable tuning bars 532,534 and 570, 572, 574, 576, 578. A curved or discontinuous groove orgrooves may be defined in the longitudinally vibratable tuning bars.

According to the embodiments shown in FIGS. 16 and 17, a through groove,a blind groove, a straight groove, a curved groove, or a groove in theform of a combination of these grooves is defined at least one of aplurality of vibratable bodies including input and output vibratablebodies.

With this arrangement, the composite longitudinal vibration mechanicalfilter has a highly accurate central frequency, improved bandpasscharacteristics, and reduced variations of the characteristics of thelongitudinally vibratable tuning bars. Composite longitudinal vibrationmechanical filters which are mass-produced have uniformizedcharacteristics and an improved quality.

FIGS. 18(a) and 18(b) show a composite longitudinal vibration mechanicalfilter in accordance with yet another embodiment of the presentinvention.

The composite longitudinal vibration mechanical filter illustrated inFIGS. 18(a) and 18(b) has an input longitudinally vibratable tuning bar632 and an output longitudinally vibratable tuning bar 634 which isidentical in shape to the input longitudinally vibratable tuning bar632. The input and output longitudinally vibratable tuning bars 532, 634are disposed in one plane and joined to each other by a pair of thincoupling elements 636, 638 made of an identity elastic material.Supporting elements 640, 642 project outwardly from the centers of theinput and output longitudinally vibratable tuning bars 632, 634. Theinput and output longitudinally vibratable tuning bars 632, 634 haverespective through holes 632a, 634a which have opening sizes smallerthan the wavelength of the longitudinal vibration of the tuning bars.The through holes 632a, 634a are defined by etching at the same timethat the longitudinally vibratable tuning bars 632, 634, the couplingelements 636, 638, and the supporting elements 640, 642 are fabricatedas a unitary structure by the photolithographic process. The otherstructural components are the same as those of the compositelongitudinal vibration mechanical filter shown in FIG. 6. Therefore,those identical components are denoted by identical reference numerals,and will not be described in detail.

Operation of the composite longitudinal vibration mechanical filtershown in FIGS. 18(a) and 18(b) is as follows:

As can be understood from the equations (3) and (4) above, the resonantfrequency F₄ of the input longitudinally vibratable tuning bar 632 andthe resonant frequency F5 of the output longitudinally vibratable tuningbar 634 are inversely proportional to the lengths L₄, L₅ of therespective tuning bars 632, 634. The accuracy of the lengths L₄, L₅ isdependent on the photolithographic technology which is employed tofabricate the longitudinally vibratable tuning bars 632, 634. Theaccuracy of the lengths L₄, L₅ cannot be achieved with sufficientlysmall error because of the thickness of the tuning bars 632, 634.Generally, the dimensional accuracy δL of the length of the tuning bars632, 634 is expressed by:

    δL=±1.5/10·t                             (29)

where t is the thickness of the tuning bars 632, 634. The dimensionalaccuracy δL does not vary greatly since the input and outputlongitudinally vibratable tuning bars 632, 634 with the through holes632a, 634a are simultaneously fabricated as a unitary structure byetching. The signs of the equation (28) remain the same as those of theequation (5). W is the width of the tuning bar and L_(M) is the lengthof the groove and/or the length where the through holes exist.

The through holes 632a, 634a defined in the longitudinally vibratabletuning bars 632, 634 will hereinafter be described.

It is assumed that the longitudinally vibratable tuning bars 632, 634have a length L (L₄, L₅ ), the through holes 632a, 634a have a width M(see FIG. 18(b)), the distribution ratio of the through holes 632a, 634ain the input and output longitudinally vibratable tuning bars 632, 634(the ratio of the sum of the areas of the through holes to the entirearea of the central portion of the longitudinally vibratable tuning bar)is indicated by γ, the input and output longitudinally vibratable tuningbars 632, 634 have a width W, and the length of the longitudinallyvibratable tuning bars 632, 634 where the through holes 632a, 634a arepresent (in the direction in which the longitudinal vibration ispropagated) is indicated by L_(M), and also that the material of whichthe longitudinally vibratable tuning bars 632, 634 are made has anaverage mass ρ.

Since the through holes 632a, 634a defined in the input and outputlongitudinally vibratable tuning bars 632, 634 are sufficiently small ascompared with the wavelength of the longitudinal vibration, the throughholes 632a, 634a do not interfere with operation of the input and outputlongitudinally vibratable tuning bars. The cross-sectional area S_(c) ofthe longitudinally vibratable tuning bars where the through holes 632a,634a are present and longitudinal vibration takes place is small becauseof the through holes 632a, 634a. The cross-sectional area S_(c) is givenby:

    S.sub.c =(W-γM)·t                           (30)

The cross-sectional area S_(d) of the longitudinally vibratable tuningbars where no through holes are present and longitudinal vibration takesplace is given by:

    S.sub.d =W·t                                      (31)

If the width W of the longitudinally vibratable tuning bars is reducedby the dimensional accuracy δL due to an etching error (overetching),then each of the through holes 632a, 634a is widened by δL. At thistime, the cross-sectional areas S_(c), S_(d) are expressed as follows:##EQU3##

The length L (L₄, L₅) of the input and output longitudinally vibratabletuning bars 632, 634 now becomes (L-δL).

The effect of an added mass on the input and output longitudinallyvibratable tuning bars 632, 634 will be considered below.

(1) When δL=0, a mass represented by {γM·(L-L_(M))·t·ρ} and commensuratewith the width W of the longitudinally vibratable tuning bars is addedto the distal ends of the input and output longitudinally vibratabletuning bars 632, 634 that have the cross-sectional area {(W-γM)·t}, andthe length of the input and output longitudinally vibratable tuning bars632, 634 is indicated by L.

(2) When δL≠0, a mass represented by {(γM+γδL)·(L-L_(M))·t·ρ} andcommensurate with the width w of the longitudinally vibratable tuningbars and a mass represented by {(-δL)·W·t·92 } and commensurate with thelength L (L₄, L₅) of the longitudinally vibratable tuning bars are addedto the distal ends of the input and output longitudinally vibratabletuning bars 632, 634 that have the cross-sectional area{(W-γM-(1+γ)δL)·t}, and the length of the longitudinally vibratabletuning bars is indicated by L (though the length is indicated by L-δL,the dimensional accuracy δL is considered as an added mass).

As a result of comparison between the equations in the cases (1) and (2)above, the mass δρ which is newly added when δL≠0 is given by thefollowing equation:

    δρ=δL·{γ(L-L.sub.M)-W}·t·.rho.·{(W-γM)·t·ρ}/{(W-γM-(1+.gamma.)δL)·t·ρ}                   (34)

If the dimensions of the longitudinally vibratable tuning bars areselected such that γ(L-L_(M))-W=0, i.e.,

    L.sub.M =L-W/γ                                       (35)

then δρ=0

even when δL≠0.

Therefore, the mass of the longitudinally vibratable tuning bars doesnot vary, and hence the resonant frequency of the input and outputlongitudinally vibratable tuning bars 632, 634 does not vary.

As described above, the through holes 632a, 634a which have an openingsize sufficiently smaller than the wavelength of the longitudinalvibration is defined in the input and output longitudinally vibratabletuning bars 632, 634. Even if the longitudinally vibratable tuning bars632, 634 do not have a sufficient dimensional accuracy, i.e., they havedifferent lengths due to an etching error which is caused when they arefabricated as a unitary structure, the central frequency of thecomposite longitudinal vibration mechanical filter does not vary and thebandpass characteristics thereof are not degraded because of the throughholes 632a, 634a which are defined by etching in the longitudinallyvibratable tuning bars 632, 634 at the same time that they arefabricated.

Another composite longitudinal vibration mechanical filter whichincludes five longitudinally vibratable tuning bars and achieves agreater amount of attenuation outside of the passband will be describedwith reference to FIG. 19.

The composite longitudinal vibration mechanical filter comprises inputand output longitudinally vibratable tuning bars 670, 678, threelongitudinally vibratable tuning bars 672, 674, 676 disposed between thelongitudinally vibratable tuning bars 670, 678, and coupling elements682a, 682b, 684a, 684b, 686a, 686b, 688a, 688b by which thelongitudinally vibratable tuning bars 670, 672, 674, 676, 678 are joinedtogether. The longitudinally vibratable tuning bars 670, 672, 674, 676,678 have plural through holes 670a, 672a, 674a, 676a, 678a definedtherein.

Supporting elements 690, 692 project outwardly centrally from the inputand output longitudinally vibratable tuning bars 670, 678, and haveouter ends secured to inner opposite edges of an outer frame 697. A pairof input piezoelectric ceramic members 699a, 699b is superposed on andfixed to the opposite surfaces of the input longitudinally vibratabletuning bar 670, and a pair of output piezoelectric ceramic members 695a,695b is superposed on and fixed to the opposite surfaces of the outputlongitudinally vibratable tuning bar 678. A feed line 691 and agrounding line 691e are connected respectively to the inputpiezoelectric ceramic members 699a, 699b, and an outlet line 693 and agrounding line 693e are connected respectively to the outputpiezoelectric ceramic members 695a, 695b.

The composite longitudinal vibration mechanical filter shown in FIG. 19operates in basically the same manner as the composite longitudinalvibration mechanical filter shown in FIG. 7.

However, use of the plural longitudinally vibratable tuning bars 670,672, 674, 676, 678 is effective in greatly reducing dimensionalvariations of these tuning bars, and in improving the passbandcharacteristics of the mechanical filter. Inputs are provided via leads691, 691e, and outputs are obtained across leads 693, 693e.

In the embodiments shown in FIGS. 18 and 19, the through holes 632a,634a and 670a, 672a, 674a, 676a, 678a are defined through thelongitudinally vibratable tuning bars 632, 634 and 670, 672, 674, 676,678, respectively. However, the invention is not limited to thosegrooves. Through holes and/or blind holes or recesses may be defined inthese longitudinally vibratable tuning bars, or two or more holes may bedefined in one or some of the longitudinally vibratable tuning bars 632,634, and 670, 672, 674, 676, 678.

As described above, the embodiments shown in FIGS. 18 and 19 reside inthat through holes and/or blind holes may be defined in at least one ofthe vibratable bodies including the input and output vibratable bodies.

With the above arrangement, the composite longitudinalvibration-mechanical filter has a highly accurate central frequency,improved passband characteristics, reduced characteristic variationsbetween the longitudinally vibratable tuning bars, provides uniformcharacteristics when it is mass-produced, and is of improved quality.

FIG. 20 shows a composite longitudinal vibration mechanical filteraccording to a still further embodiment of the present invention. Thecomposite longitudinal vibration mechanical filter has coupling elements736, 738 which are positioned near the distal ends of input and outputlongitudinally vibratable tuning bars 732, 734, i.e., in regions wherethe longitudinally vibratable tuning bars are displaced to a largeextent in the direction in which the longitudinal vibration takes place.The vibration is propagated (coupled) through the coupling elements 736,738 as a transverse wave, i.e., so-called flexural vibration, so thatspurious responses are reduced and the passband characteristics areimproved.

The composite longitudinal vibration mechanical filter shown in FIG. 20is substantially identical to the composite longitudinal vibrationmechanical filter shown in FIG. 9.

Reduction of spurious responses with the structure shown in FIG. 20 willbe described below.

The input and output longitudinally vibratable tuning bars 732, 734 aredisplaced to a greater extent at their distal ends in the direction inwhich the longitudinal vibration takes place. Displacement of thelongitudinally vibratable tuning bars 732, 734 in a direction normal tothe longitudinal direction is greater at the center of thelongitudinally vibratable tuning bars 732, 734. The displacement by thelongitudinal vibration of the input longitudinally vibratable tuning bar732 in the direction of the longitudinal vibration, and the displacementthereof in the direction normal to the longitudinal vibration, aretransmitted (coupled) to the output longitudinally vibratable tuning bar734 via the coupling elements 736, 738.

At this time, not only the displacement normal to the longitudinalvibration is coupled to the longitudinal vibration of the outputlongitudinally vibratable tuning bar 734 through the coupling elements736, 738, but also vibration in another mode is coupled to thelongitudinal vibration of the output longitudinally vibratable tuningbar 734. Therefore, spurious responses are of a large value,deteriorating the filter characteristics. The vibration normal to thelongitudinal vibration is propagated mainly as a longitudinal wave inthe coupling elements 736, 738, and the displacement in the direction ofthe longitudinal vibration produces smaller spurious responses than thedisplacement normal to the longitudinal vibration as it is coupled tothe longitudinal vibration of the output longitudinally vibratabletuning bar 34 via the coupling elements 736, 738. The longitudinalvibration is propagated as flexural vibration in the coupling elements736, 738.

The coupling elements 736, 738 are disposed in the regions where thedisplacement in the direction of the longitudinal vibration is large,near the distal ends of the input and output longitudinally vibratabletuning bars 732, 734. The input and output longitudinally vibratabletuning bars 732, 734 are coupled to each other by the flexural vibrationvia the coupling elements 736, 738. Accordingly, spurious responses arereduced, and the passband characteristics are improved.

The displacement of the input longitudinally vibratable tuning bar 732in the direction of the longitudinal vibration is larger at its distalends, and is represented as a function of the position in the directionof the longitudinal vibration. In order to provide desired frequencycharacteristics and reduce dimensional variations of the longitudinallyvibratable tuning bars, it is necessary to uniformize the amount ofcoupling of the output longitudinally vibratable tuning bar 734 to theinput longitudinally vibratable tuning bar 732. The coupling elements736, 738 should be positioned relatively to the input longitudinallyvibratable tuning bar 732 as constantly as possible. More specifically,the relative position between the input and output longitudinallyvibratable tuning bars 732, 734 and the coupling elements 736, 738 canbe rendered constant by fabricating the input and output longitudinallyvibratable tuning bars 732, 734 and the coupling elements 736, 738 froma single sheet by etching according to the photolithographic process.

Another composite longitudinal vibration mechanical filter whichcomprises five longitudinally vibratable tuning bars and providesincreased frequency attenuation outside of the passband is illustratedin FIG. 21.

The composite longitudinal vibration mechanical filter shown in FIG. 21comprises input and output longitudinally vibratable tuning bars 770,778, three longitudinally vibratable tuning bars 772, 774, 776 disposedbetween the longitudinally vibratable tuning bars 770, 778, and couplingelements 782a, 782b, 784a, 784b, 786a, 786b, 788a, 788b by which thelongitudinally vibratable tuning bars 770, 772, 774, 776, 778 are joinedtogether.

Supporting elements 790, 792 project outwardly centrally from the inputand output longitudinally vibratable tuning bars 770, 778, and haveouter ends secured to inner opposite edges of an outer frame 797. A pairof input piezoelectric ceramic members 799a, 799b is superposed on andfixed to the opposite surfaces of the input longitudinally vibratabletuning bar 770, and a pair of output piezoelectric ceramic members 795a,795b is superposed on and fixed to the opposite surfaces of the outputlongitudinally vibratable tuning bar 778. The composite longitudinalvibration mechanical filter also has a feed line 791 and a groundingline 791e which are connected respectively to the input piezoelectricceramic members 799a, 799b, and an outlet line 793 and a grounding line793e which are connected respectively to the output piezoelectricceramic members 759a, 759b.

The composite longitudinal vibration mechanical filter shown in FIG. 21operates in basically the same manner as the composite longitudinalvibration mechanical filter shown in FIG. 20.

With the plural longitudinally vibratable tuning bars 770, 772, 774,776, 778 employed, dimensional variations of these tuning bars arereduced, and the passband characteristics of the mechanical filter areimproved.

According to the above embodiments shown in FIGS. 20 and 21, thecomposite longitudinal vibration mechanical filter for delivering asupplied high-frequency signal in a predetermined frequency rangeincludes a plurality of longitudinally vibratable bodies including inputand output vibrarable bodies for receiving and delivering thehigh-frequency signal, the vibratable bodies being longitudinallyvibratable in a range close to the passband of the mechanical filter, aplurality of piezoelectric members superposed on the input and outputvibratable bodies and including electrodes to which conductors areconnected, and a plurality of coupling elements disposed between ends ofthe vibratable bodies and coupling them through. flexural vibration.

With such an arrangement, the composite longitudinal vibrationmechanical filter has a highly accurate central frequency, improvedpassband characteristics, provides uniform characteristics when it ismass-produced, and is of improved quality.

Although certain preferred embodiments have been shown and described, itshould be understood that many changes and modifications may be madetherein without departing from the scope of the appended claims.

What is claimed is:
 1. A method of manufacturing a compositelongitudinal vibration mechanical filter that vibrates at a preselectedcentral frequency in a high frequency range, wherein the filtercomprises:a plurality of vibratable bodies including input and outputvibratable bodies with piezoelectric members superposed thereon, each ofsaid vibratable bodies having a predetermined length; coupling elementswhich couple the vibratable bodies to each other; supporting elementsprojecting respectively from the input and output vibratable bodies; anda holder to which the supporting elements are attached, said methodcomprising the steps of:forming, at the same time, an integral bodycomprising said plurality of vibratable bodies including at least saidinput and output vibratable bodies, said coupling elements, saidsupporting elements, said holder and frequency deviation eliminatingmeans on at least one of said vibratable bodies, said frequencydeviation eliminating means being formed so as to include at least oneresonant frequency adjusting finger for eliminating a deviation of thecentral frequency of said filter; superposing said piezoelectric membersfixedly on the input and output vibratable bodies in sandwichingrelation thereto; and connecting electrodes to said piezoelectricmembers.
 2. A method of manufacturing a composite longitudinal vibrationmechanical filter according to claim 1, further including a step ofmodifying a size of said at least one resonant frequency adjustingfinger of said frequency deviation eliminating means after said step ofconnecting electrodes to said piezoelectric members.
 3. A method ofmanufacturing a composite longitudinal vibration mechanical filteraccording to claim 1, further comprising the steps of:operating thefilter, comparing an output of the filter to a desired central frequencydeviation, and producing a comparison output based on a result of saidcomparing; and modifying a dimension of said at least one resonantfrequency adjusting finger of said frequency deviation eliminating meansresponsive to said comparison output so that said output of said filtermeets said desired central frequency deviation.
 4. A method ofmanufacturing a composite longitudinal vibration mechanical filteraccording to claim 1, wherein said steps of forming said integral body,superposing said piezoelectric members and connecting electrodes to saidpiezoelectric members are performed sequentially in the recited order.5. A method of manufacturing a composite longitudinal vibrationmechanical filter according to claim 1, comprising providing saidvibratable bodies which vibrate at a frequency so as to produce a highfrequency output signal of at least 455 kHz.
 6. A method ofmanufacturing a composite longitudinal vibration mechanical filter thatvibrates at a preselected central frequency in a high frequency range,said filter including a plurality of vibratable bodies having input andoutput vibratable bodies with piezoelectric members superposed thereon,coupling elements for coupling the vibratable bodies to each other,supporting elements projecting respectively from the input and outputvibratable bodies, and a holder to which the supporting elements areattached, said method including the steps of:fabricating said vibratablebodies to have a predetermined length, said vibratable bodies beinglongitudinally vibratable along said predetermined length thereof;superposing said piezoelectric members fixedly on the input and outputvibratable bodies in sandwiching relation thereto; forming at least oneresonant frequency adjusting finger on at least one of said vibratablebodies; and physically modifying said at least one resonant frequencyadjusting finger for eliminating a deviation of the central frequency ofsaid filter.
 7. A method of manufacturing a composite longitudinalvibration mechanical filter according to claim 6,further comprising thesteps of operating the filter, comparing an output of the filter to adesired central frequency deviation, and producing a comparison outputbased on a result of said comparing; and wherein said step of physicallymodifying said at least one resonant frequency adjusting finger includesmodifying a dimension of said at least one resonant frequency adjustingfinger responsive to said comparison output so that said output of saidfilter meets said desired central frequency deviation.
 8. A method ofmanufacturing a composite longitudinal vibration mechanical filteraccording to claim 6, comprising providing said vibratable bodies whichvibrate at a frequency so as to produce a high frequency output signalof at least 455 kHz.